The present invention relates to an image recording system, such as an electrophotographic printer, a copying machine, etc.; and, more particularly, the invention relates to a developing system using magnetic developing agents.
The image recording system, such as used in an electrophotographic printer, a copying machine, etc., forms an electrostatic latent image having a preset potential VR (and the non-image area has a preset potential VO) on an image carrier called a photoconductive drum, which rotates in one direction only. A developing agent called a toner is supplied from a developing unit to the drum to render the latent image visible, and the toner image is then transferred onto recording paper. Conventionally this kind of electrophotographic image recording system mostly employs a developing unit which uses a two component developer containing both a toner and a magnetic powder called a "carrier".
Usually, this kind of developing unit stirs the two component developer in a storage tank. The toner and the carrier in the storage tank vigorously rub against each other and are respectively charged to preset magnitudes. This charged developing agent then is fed from the developing agent tank to the developing means in the form of "developing rollers" each of which carries a plurality of magnets on the circumference thereof. The charged developing agent is attracted to the developing rollers and carried on the surface of the developing rollers.
The developing agent on each developing roller is leveled by a parting plate called a "doctor blade." The polarities of the magnets on the rotational upstream and downstream sides relative to the doctor blade are reversed to increase the transferability of the developing agent on each developing roller.
Next, the developing agent on the developing roller which is leveled by the doctor blade is rotationally carried into contact with the surface of the photoconductive drum. At the same time, a bias potential (hereinafter called a "developing bias") VB is applied to the developing rollers to transfer only toner onto the latent image on the surface of the photoconductive drum. Consequently, the latent image on the photoconductive drum is rendered visible.
There are three ways to make the developing agent on the developing rollers come in contact with the surface of the photoconductive drum: a first way involves rotating the developing roller in the same rotational direction as said photoconductive drum (hereinafter termed "forward rotation") to transfer the developing agent; a second way involves rotating the developing roller in a rotational direction opposite to the direction of rotation of said photoconductive drum (hereinafter termed "backward rotation") to transfer the developing agent; and a third way involves use of both a forward-rotating developing roller and a backward-rotating developing roller to transfer the developing agent.
In the above-mentioned developing techniques, the ratio of the rotational speed of the developing roller to the rotational speed of the photoconductive drum (hereinafter termed "peripheral speed ratio") is generally greater than 1. In the developing system using a two component developer, more particularly, in the developing system having a backward-rotating developing roller on the rotational upstream side of the photoconductive drum, as disclosed in Japanese Non-examined Patent Publication No. 02-8308 (1990), a forward-rotating developing roller on the rotational downstream side, which is very close to the backward-rotating developing roller, and a doctor blade between said developing rollers, the developing agent is transferred in a mass towards the doctor blade, divided into two parts for the two developing rollers by the doctor blade, and then is applied to the developing rollers through a gap between the doctor blade and each developing roller. In this method, the doctor blade is placed with its top edge facing the photoconductive drum (on the downstream side of the developing agent relative to the central axes of the developing rollers). Each end of the base side of the doctor blade and the surface of each developing roller forms a gap into which the developing agent is filled. Consequently, the developing agent has a high bulk density before being conveyed to each of the developing rollers. However, in this case, the excess developing agent which does not go through the doctor gaps remains at the doctor blade in a narrow space between the developing rollers. As a result, the developing agent undergoes unwanted stresses and, consequently, the service life of the developing agent becomes shorter.
A proposed method disclosed in Japanese Non-examined Patent Publication No. 07-160123 (1995) comprises placing the doctor blade further away from the surface of the photoconductive drum (on the upstream side of the developing agent relative to the central axes of the developing rollers) and dividing the flow of the developing agent through these wider doctor gaps.
A method which involves placing the doctor blade on the upstream side of the developing agent relative to the central axes of the developing rollers (further away from the surface of the photoconductive drum) provides a less stable supply of the developing agent to the developing rollers than a method which involves placing the doctor blade on the downstream side of the developing agent relative to the central axes of the developing rollers (nearer to the surface of the photoconductive drum) because the higher filling pressure of the developing agent is not available.
In a developing system in which the front end of said doctor blade is placed on the downstream side of the developing agent relative to a line through the central axes of the developing rollers, the excess developing agent which does not go through the doctor gaps remains at the doctor blade in a narrow space between the developing rollers. As a result, the developing agent undergoes unwanted stresses and, consequently, the service life of the developing agent becomes shorter.